1. Field of the Invention
This invention relates to a magnetic resonance imaging apparatus which obtains figure information such as a tomography of the subject, and functional information such as spectroscopic information, making use of a magnetic resonance (MR) phenomenon.
2. Description of the Related Art
A magnetic resonance phenomenon is such that an atomic nucleus with some spin and magnetic moment placed in a static magnetic field resonantly absorbs and radiates only electromagnetic waves of a specific frequency. The nucleus resonates at an angular frequency of .omega..sub.o expressed by the following equation (.omega..sub.o =2.pi..gamma..sub.o, .gamma..sub.o =Larmor frequency): EQU .gamma..sub.o =.gamma.H.sub.o
where .gamma. is the gyromagnetic ratio peculiar to the type of atomic nucleus, and H.sub.o is the intensity of the static magnetic field.
An apparatus based on the above-mentioned principles processes induced electromagnetic waves of the same frequency as that of the absorbed electromagnetic waves after the aforementioned resonant absorption, and thereby obtains diagnostic information reflecting MR parameters (including atomic nucleus density, longitudinal relaxation time T1, transverse relaxation time T2, flow, and chemical shift), such as sliced images of the subject, with no invasion.
The acquisition of diagnostic information by magnetic resonance is achieved by exciting all parts of the subject placed in a static magnetic field and collecting the signals. Because of restrictions on apparatus configuration and clinical request for images, actual apparatuses excite a specific part of the subject and collect the signals.
In this case, a specific part to be imaged is generally a sliced part having a particular thickness. Magnetic resonance signals (MR signals), including echo signals and FID signals from the sliced part, are collected by encoding data many times. By restructuring these groups of data items by, for example, a two-dimensional Fourier transformation method, laminograms (sliced images) of the specific sliced part are created.
Such a magnetic resonance imaging apparatus is installed in the clinical field as a sophisticated medical diagnostic instrument which, almost only by electrical operations, provides various pieces of diagnostic information reflecting parameters peculiar to a magnetic resonance phenomenon, including nuclide, atomic nucleus density, relaxation time, pulse repetition time, echo time, MR imaging part, and parameters determining image processing.
With a resonance imaging apparatus of this type, MR imaging is effected as follows. The MR imaging target part of the subject is set in a specified position within a gantry. After entering relevant particulars including patient registration, the operator selects an RF coil and a matrix according to diagnostic items and further selects MR imaging parameters including nuclide, atomic nucleus density, relaxation time, pulse repetition time, echo time, pulse shape, and pulse-sequence.
Furthermore, the operator also selects image processing parameters, restructuring parameters, and output parameters. After those parameters have been selected, the apparatus is ready to scan. When the scan button is operated, a predetermined pulse-sequence is repeated by means of a sequencer. The sequentially collected data items are stored in a memory in the computer system. Therefore, these data items are sent to the restructuring unit in the computer system, which restructures them to form image data. The operator then selects one of image display, imager output, and image storage as the occasion demands, and at the time of diagnosis, reads images on the display or films.
In the above-mentioned MR imaging procedure, the operator sets parameters so that images suitable for diagnosis complying with the expected diagnostic items may be obtained. However, when the obtained image is unsatisfactory or when the doctor wants to diagnose from another viewpoint, another picture needs to be MR imaged by repeating the aforementioned MR imaging procedure. For instance, the doctor may judge that an axial picture obtained by MR imaging a certain portion of the head would not provide good diagnostic information, and therefore may desire another MR image of the portion to form a sagittal picture.
As described above, with such a conventional apparatus, the MR imaging procedure must be repeated until the desired image is obtained. In this case, when what is diagnosed is a still object, there is no problem with images obtained at long intervals. However, when what is diagnosed is a moving object, images obtained at long intervals prevent pathologic analysis from being performed satisfactorily. For instance, when the movement of a knee or a joint needs to be diagnosed dynamically, or when the flow from when a contrast medium is injected until it is excreted needs to be diagnosed dynamically, various types of images can be obtained. However, because the MR imaging procedure must be repeated every time each image is formed, it takes a long time to produce each image. This prevents a dynamic diagnosis from being made actually, which becomes a problem.
Such a problem stems from the fact that once the conventional apparatus has set parameters, it cannot effect MR imaging using different parameters unless the MR imaging procedure based on the present parameters has been finished.